Process and apparatus for the catalytic cracking of a hydrocarbon oil



E. F. CHANDLER PROCESS AND APPARATUS FOR THE CATALYTIC CRACKING OF AHYDROCARBON OIL Filed March 17, 1943 2 Sheets-Sheet 1 July 8, 1947.

INVENTOR w a July 8, 1947. E. F. CHANDLER PROCESS AND APPARATUS FORCATALYTIC.

CRACKING .OF A HYDROCARBON OIL 2 Sheets-Sheet 2 Filed March 17, 1945 vINVENTO W Patented July 8, 1947 UNITED STATES PATENT OFFICE PROCESS ANDAPPARATUS FOR THE CATALYTIC CRACKING OF A HYDRO- CARBON OIL 2 Claims.

This invention pertains to methods of, and apparatus for, treatingmineral oil. 'Ih'e invention further relates to methods of, andapparatus for, improving the yield of desirable products obtained frompetroleum hydrocarbons. More specifically, the invention provides animproved process for the conversion of higher boiling fraction intolower boiling constituents suitable for use in the production of motordistillates, motor fuels, and the like. Preferably, the desiredmolecular rearrangements of the oil compositions are brought aboutthermally, in the presence of a suitable catalyst. The reactions orinter-actions are desirably caused to take place in the vapor phase, andwith or without pressure within the conversion zone. However, thepresent invention is applicable to operations in the liquid or vaporphase, and in the absence of a catalytic agent, chemical promotor, orthe like.

An important feature of the invention resides in preparing the oil thatis fed to the process in such a manner as to insure the deliveryto theconversion zone of an optimum composition selectively prepared for themost economical and efiicient treatment under the controlled conditionswhich have been predetermined and established within the conversionzone. The original oil charged to the process, for example, may becomposed of fractions having boiling points ranging from approximately96 degrees F., to possibly 790 degrees F., or thereabouts.

In its simplest form, the present process includes the step of dividingthe charging oil into a plurality of oil streams, each comprising asubstantially related but relatively narrow range of boiling points. Forexample, stream (A) may be composed of the fractions ranging from 96deg. F., to 342 deg. F. Stream (B) might contain the fractions havingboiling points ranging from about 381 deg. F. to 453 deg. F. Stream (C)may comprise the fractions boiling at from 586 deg. F., to 688 deg. F.,and stream (D) might be composed of the fractions between 714 deg. F.and 790 deg. F.

For the purpose of establishing the characteristics of the severalstreams, as above, the original charging oil may be vaporized anddelivered to a suitable fractionating tower which is adapted to deliver,as side streams, the cuts indicated as A, B, C and D, in a manner Wellknown to the art. These streams, in turn, preferably are vaporized andthe vapors separately supplied to individual conversion zones whereinthe vapors ar submitted to treatment which has been selectivelyestablished as most suitable for each set of stream characteristics. Bythis means, a method is afforded for the continuous treatment of aspecific charging stock, and at all times during the operation, eachfractional group, composing the whole, may be kept under close processcontrol with respect to temperature, pressure, etc.; individualconversion zone conditions may be selectively modified and adjusted,manually or automatically, during the operation.

The vapors issuing from each of the several conversion zones may beindividually dephlegmated for the purpose of condensing and collectingthe lower boiling hydrocarbons produced, or the vapors issuing from theseveral conversion zones may b all delivered to a single dephlegmatingor fractionating tower, and the lower boiling hydrocarbons resultingfrom th several individual conversion reactions mixed and. recovered.Preferably, while the lower boiling hydrocarbons would logically beremoved after each pass through a conversion zone, the heavierhydrocarbons which condense in the dephlegmator or fractionating columnare re-cycled for further treatment. Depending upon the method ofoperation and/ or upon the results desired, the hydrocarbon to bere-cycled may be delivered to, and mixed with, the original chargingoil; may then be selectively again passed through the same conversionzone as an addition to the hydrocarbons being delivered thereto, or there-cycle stock from one conversion zone operation may be delivered to,and re-cycled through, one of the other conversion zones.

For example, one section of the system, may be operated continuously onvirgin charging stock to produce only converted or cracked hydrocarbonsof a certain type, while the re-cycle stock from this operation isblended with, and passed to, another conversion zone to produce amixture of cracked products, etc. Preferably, as pointed out, the vaporsare thermally treated in the presence of a suitable catalytic agent oragents within the conversion zones. It will be understood that anysuitable catalyzer or chemical promotor best suited for producing theparticular results desired may be employed.

It has been pointed out herein that the process gives good results whenoperated in the vapor phase, without the aid of a catalytic agent, ascompared with the usual vapor phase operating methods. The improvedsystem of selectively treating the oil results in increased yields oflower boiling hydrocarbons with reductions in the generation of fixedgases and reduced deposits of troublesome carbon. However, in thepreferred form, the invention is particularly important as affording animproved method of operating processes in which catalytic agents areemployed. In this connection, the several advantages of catalytichydrocarbon conversion are fully realized. The process may beeconomically and efficiently operated at relatively low temperatures,the average of the several conversion zOne temperatures ranging between800 and 950 deg. F., more or less. An opportunity for a more completecatalytic action i afforded without increasing the over-all speed of theprocess, and losses due to under and over treating certain portions ofthe charge may be practically eliminated.

By maintaining optimum conditions within the conversion zone for themost eficient treatment of a certain hydrocarbon group, selectivelyassur- 4 or carbon which is freed, due to slight maladjustment of theseveral predetermined optimum conditions of either the group offractions being fed to the conversion zone, or of conditions within thezone itself, will exist in the nascent state and is readily re-combinedunder the thermocatalytic conditions of the conversion zone to form newhydrocarbons, with a minimum loss in the form of non-condensable gaseousproducts. This building-up effect can be further enhanced,

if required, by enriching the oil being delivered to a particularconversion zone, by the addition ing the presence of this group as theoptimum charge being delivered to the particular conversion zone,improvements are obtained in the qualityand the quantity of the yield oflow boiling hydrocarbons which indicate new and important advances inthe art of catalytic hydrocarbonrconversion. The method of selectivelystaging the thermo-catalytic treatment of the charging stock inpredetermined cuts having substantially optimum characteristics, withrespect to the particular treatment within each said stage, affords adegree of flexibility of process control far beyond anything heretoforeobtainable in thoseprocesses in which it is customary to subject acomparatively wide hydrocarbon fractional range to the relativelylimited reactive scope of the catalyst.

It is well known that when hydrocarbon vapors comprising a wide range offractions of different boiling points. are contacted with a catalyzerwithin a heated zone, only a limited few of these fractions fall withinthe optimum range of thermo-catalytic conversion activity, with the result thatany fractions above or below this relatively limited range donot respond to the'desired treatment. The more refractory constituentsmay pass through the zone substantially unchanged, while the lessrefractor fractions are converted largely into free hydrogen andcokeforming products of carbon. It is to avoid these and otherwell-known objections to processes of the presently used types that thismethod of multi-selective conversion has been devised.

While the process herein described has been designed to overcome theobjections outlined, actual experience has shown that the broad methodemployed aifords inherent operating advantages heretofore obtainableonly by the employment of separate, independent means. For example,experience has shown that no matter how carefully the effort is made toadjust the conditions of optimum charging stock to predetermined optimumthermo-catalytic treating conditions, there will be some products ofside reactions. The present process, by the method described, reducesthe results of these eflectsto the minimum, but which neverthelessresults in the release of hydrogen from some of the hydrocar bons with aresulting dropping out of carbon. However, it has been found that in thepresent process, losses due to these causes are practically absent inthe conversion zone handling the least refractory group of hydrocarbonfractions, probably because of the greater ease with which optimumoperating conditions can be established and maintained.

In the conversion zones which handle the more refractory groups ofhydrocarbons, any hydrogen thereto of suitable hydrocarbon fractionsselectively obtained from the virgin stock being initially delivered tothe process, and preferably having a sufficiently high hydrogen contentto meet the deficiency mentioned. A similar result may also be obtainedby utilizing a re-cycle stock from one of the conversion zones found topossess the desired composition.

As in thermal cracking, long-chain hydrocar" bons above the gasolineboiling range are broken down into smaller molecules which boil withinthe gasoline range. Cracking, which takes place in the presence of acatalyst, is generally accomplished at relatively lower temperatures,and either at atmospheric or relatively low pressures above atmospheric.A suitable catalyst serves not only to facilitate the rupturedhydrocarbon chains, but also controls the mechanism of their destructionalong certain paths to form less olefinic materials and more materialsof the branched-chain and aromatic types, which dis- 7 play improvedanti-knock characteristics.

In catalytic processes of the usual type, a gasoline yield of about 45%is obtained, based on the charge to the catalyst. Also, from practicallyany stock employed, the octane number will range from about 71 to 81. Inoperating the process herein described, the average yield isconsiderably increased, ranging between 45% and 52%, and having anoctane number of the order of from 78 to 82. The increase both in theyield of gasoline and anti-knock rating is attributed to the greatereconomy and eificiency of the selective system of operation wh chlikewise affords other improvements in performance, as, for example, arelatively longer cycle of continuous, uninterrupted operation and areduction of the usual quantity of fixed gas andlow-grade end products.

Further economies and advantages in the overall performance may beobtained when the presently-described process, operating preferablycatalytically, is employed in conjunction with the system described inmy co-pending application Serial No. 455,553, filed August 21, 1942,Conversion of Petroleum, in which case the heavier liquid hydrocarbonsresulting in the first said process would be delivered to the secondsaid process for further treatment and the production of additionallower boiling constituents by the method described therein. This secondprocess,

may be operated as a straight vapor: phase, thermal-cracking unit, or,as pointed out therein, as a catalytic conversion process.

The operation of this process, in the broad sense, is not necessarilypredicated on the use of any specific form or type of catalyzer, manykinds of which are well known as suitable for employment in connectionwith the treatment of hydrocarbons. It is desirable, however, in view ofthe method described in which relatively specific groups of hydrocarbonsmay be selectively set apart and submitted to predetermined,

closely-controlled conditions, that the catalyzer decided upon, or theconditions under which a specific catalyzer is used, be governed by afull consideration of the characteristics of the hydrocarbons to betreated within a given reaction zone and with careful respect to theultimate results to be obtained.

Probably, for the first time, this pro zess affords the means forcontinuously running a stream of hydrocarbon oil of highly complexmolecular structure, under practically ideal conditions in so far as theseveral groups of hydrocarbons, separated from the whole, while actuallysubject to treatment, represent substantially optimum qualitiesselectively adapted to the treating range of a given set of thermal andchemical conditions called for by the catalyzer or chemical promotordecided upon. For example, in the operation of the process for theconversion of higher boiling hydrocarbons into hydrocarbons boilingWithin the gasoline distillate range, good results have been obtained bythe use of a catalyst in pellet form so packed into the conversion tubesas to allow the free passage therethrough of the hydrocarbon vapors.These pellets were composed of kieselguhr to afford a base havingultramicroscopic capillary pores combined with an active agent such ashydrogen-reduced. iron particles. Mixed, for example, with sodiumhydrate and water to form a paste, the pellets are formed and baked tohardness. When completed, they are extremely porous and absorptive atelevated temperatures and in the presence of hydrocarbon vapors.

The composition, particularly with respect to the active material,responds to wide modification to meet any required catalytic or chemicalpromotional effect. During these operations, good results were obtainedusing the same type of catalyzer in each selective stage, by modifyingthe temperature and timing of the reaction period in favor of theparticular group of hydrocarbons being passed therethrough. Thecatalyzer has been found to be highly resistant to poisoning, willoperate efficiently over long periods, and is easily and quicklyre-activated in an atmosphere of super-heated steam. It should beunderstood that all phases of operation of the process should beaccurately and closely governed. Careful control of. temperatures isimportant and should be automatically maintained. The rate and quantityof oil feed should be carefully regulated, both with respect to thecharging stock as well as the proportioning and distribution of sidestream and re-cycle stocks.

Usually, in operating cracking processes of the continuous type, the oilis pumped through the system at a relatively constant, uniform rate.Experience with the present system has shown that while this method maybe employed, certain important advantages can be obtained by pumping theoil through the system in uniform impulses or waves. In this manner, adistinct dwell is afforded to the vapors in the conversion zones, andwhile they are in contact with the catalytic agents. This dwell, whilerelatively brief, affords an opportunity for the completion of thereaction period, the products of the reaction being ejected, as it were,by the next impulse bringing fresh hydrocarbons into the conversionzone. The timing of the impulses may be such that the over-all rate offeed is not reduced.

In the drawings, which illustrate schematically several alternativeforms of the method of the present invention,

Fig. 1 shows schematically one arrangement of apparatus for carrying outthe present invention; and

Figs. 2, 3 and 4 show alternative forms.

In Fig, l the charging stock is delivered by pipe 2 to the pre-heatercoil 3 positioned at the upper end of tower ll. From the lower end ofpre-heater coil 3, the oil is delivered to vaporizer 4 by pipe 5. Thenby pipe 6 the charging stock is conveyed to expansion chamber '1 at thelower end of tower H. The lighter non-condensing products pass throughopening 8 in a vapor state, and some of the higher boiling fractionscondense in tower H, fall back upon tray 9, and are delivered by pipe Itto a heater coil 82 where the same are vaporized and delivered to theheated catalyzer zone 3. The products which are converted in zone l3 aredelivered by pipe M to heat exchanger l5 Where, preferably by indirectheat exchange, as shown, the oil passing through pipe l is vaporized.Lower boiling products escape from tower ll, through pipe l0 and aredelivered to tower ll at a point above tray l8 where they are filmed onsuitable plates !9.

The heavier products collect on the tray l8 and are drawn off throughpipe 20, and thus may be passed through heat exchanger 2! to be carriedoff by pipe 22 to oil heater or vaporizer 23.

From this vaporizer, the vapors are delivered to the, second catalyzerzone 2%. The heated, converted products of this reaction pass throughpipe 25 to heat exchanger l5, which also supplies heat to coil 4. Themixed and blended heated products from heat exchanger 55 then passthrough pipe 26 into the lower end of tower H, and thence upwardlythrough perforated tray I8. The lighter vaporous constituents issuethrough tray I 8 to heat and vaporize the oil being delivered into thetower by pipe l6, and are aided by the filming action of plates I9. Thelower boiling hydrocarbons which may be within the motor distillaterange leave the tower l! in vapor form through pipe, 28, and thence passthrough suitable condensing means 29 in a manner well known in the art;

In this: arrangement, it will be noted that conversion zone i3 isoperated exclusively upon by drocarbons which have been condensed out ofthe originalv charging stock and hence are substantially virginuncracked oil, whereas conversion zone 24 is operated on a mixture ofcracked and uncracked hydrocarbons. It is understood that some thermalcracking may take place during the vaporization within the tube 4. Theproducts which, pass through zone l3 being less refractory, may besubjected to a milder thermo-catalytic action, while the productspassing through zone 24' being more refractory, may be subjected to amore intense thermo-catalytic action. For example, during the passagethrough zone l3 of the vapors from the oil vaporizing coil l2,sufficient heat would be delivered thereto by a furnace (not shown) toafford a vapor phase conversion temperature of the order of from 850 to950 deg. 9., while the heat delivered to the oil vapors passing throughzone 24 would be sufficient to raise their temperature to the order of1,000 to 1,050 deg. F., the exact temperatures depending, as hereinpointed out, upon the kind of oil being treated, the type of catalyzeremployed (if any), and other recognized factors. The process has beensuccessfully operated at substantially atmospheric pressure. Pressure,however, may be employed should the kind of oil and other operatingconditions so require. Pressures of the order of from 2 to 5 atmosphereshave been employed, and higher pressures are obviously possible.Preferably, only such pressure is retained on the system as results fromthe oil being delivered thereto through pipe 2 by suitable pumping means(not shown), which may not be substantially above atmospheric, as setforth. An important advantage of the method herein described employingtwo or more selectively adjusted thermo-catalytic Zones to which thevapors of predetermined oil cuts are delivered, resides in the fact thatthe cracking action in each zone is under conditions conducive to themaximum efliciency. There is a minimum of fixed gas and polymerizedmaterial formed. Depending upon the kind of oil being treated, the feedrate may be of such order that the vapors are under treatment within therespective zones for from 5 to 300 seconds, more or less. The conversionin each vapor phase zone is conducted under conditions ascertained bytest or experience to be best for the particular cuts, and may be soadjusted as to produce cracked products having an average molecularweight ranging from approximately 30% to 75% of the average molecularweight of the out from which the cracked products are derived. In thepreparation of the cuts for thermocatalytic action in the severalselectively adjusted zones, suitable tubular heaters, I2 and I3 forexample, may be employed in which said oil cuts are subjected preferablyonly to sufficient heating to vaporize the oil substantially withoutcracking the same. preferably preheated and vaporized at temperaturescalculated to produce a minimum of precracking. The presence of somevirgin oil constituents in the vapors which are subjected to moreintense action in zone 24 affords, in the presence of a suitablecatalyst, the slight excess of hydrogen that may be required to maintaina hydrocarbon molecular balance and prevent the undue release of carbon.

In the arrangement shown in Fig. 2, the charging stock enters by a pipe35, passes through preheater coil 36 in tower 4|, and then by pipe 31enters the vaporizing coil 38 in heat exchanger 42. The oil leaves thevaporizing coil by a pipe 39 and enters as vapor in a suitablefractionating tower 40 which is equipped to develop a plurality of sidestreams 44, 45 and 46, etc. To indicate the possibility of adjusting thecharacteristics of said side streams, valve outlets 41, 48 and 49 areconnected with suitable tray levels within tower 40. It is understoodthat the purpose of these valved outlets is to permit the selection ofside streams of the desired boiling point groups, For the purpose ofremoving from tower 40 uncondensed vapors of lower boiling hydrocarbons,pipe 50 is shown as leading to a suitable condenser 5|. However, itwould be understood that these latter products may, if desired, be fedto a suitable reforming unit for the purpose of improving the anti-knockrating of the distillate and recovery of other desired products.

Side stream 44 is fed to a suitable oil-vaporizing furnace coil 52, andthen to conversion zone 53 where it is delivered by pipe 54 to heatexchanger 42. Likewise, side stream 45 is fed to vaporizing coil 56,then to conversion zone 51, and then by pipe 58 to heat exchanger 42.Side stream 46 is similarly fed to heater 60, thence to conversion zone6|, and by pipe 62 the heated conversion products are delivered to heatexchanger 42. In this manner, the heated products of the severalreaction zones blend and serve to The incoming charging stock also is'be employed for this purpose.

8. heat the oil in coil 38 and also the oil in coil 63 as the productspass therethrough and leave the pipe 64, which conveys the oil intotower 4|. The heavier hydrocarbons which condense and collect in thelower part of tower 4| are removed through pipe 65, and are thendelivered to coil 63 wherein they are vaporized. Leaving coil 63, thevapors pass to tower 40 by pipe 66, in which latter tower they mix andblend with the vapors delivered by pipe 39.

Tower 4|, like tower 40, is provided with an outlet pipe 61 by means ofwhich the lower boiling hydrocarbons may be removed, condensed andcollected, or otherwise treated as desired. In this arrangement, theheating furnaces and sources of heat for coils 52, 56 and 60, being wellknown in the art, have been omitted. Likewise, the means for selectivelyheating the conversion zones 53, 51 and 6| are not shown. It isunderstood, of course, that any well known means may The valved outlet69 at the lower end of tower 4|] may serve to remove the end products,preferably in liquid form, from the system.

In the arrangement shown in Fig, 3, the charging stock is fed throughpipe 10 and enters oilheating furnace 1!, for which 12 represents asuitable source of heat for vaporizing the oil within the furnace coil,and from which, by means of pipe 13, the vapors are discharged intotower 14. This tower may be of any suitable type and is adapted todeliver the selectively chosen side streams 15, 16 and 11, which arecomposed of hydrocarbons condensed at different levels in thefractionating system of the tower. These side streams are delivered,respectively, to the vaporizing coils 18, 19 and 80, and from thesecoils the vaporized products are delivered to the conversion zones 8!,B2 and 83, respectively. It will be understood, of course, that theseconversion zones are heated or are otherwise selectively conditioned forthe optimum handling of the respective hydrocarbon groups being passedtherethrough.

By means of the delivery pipes 84, and 8B, the heated products of theseveral conversion zones are delivered, respectively, to the expansiontowers 81, B8 and 89. At the top of each of these expansion chambers ortowers, there are provided take-off means 95, and 91, respectively. Inthis manner, vapors of lower boiling hydrocarbons may be removed fromsaid towers, the means preferably being such that the same may beadjusted to deliver simultaneously from all of such towers, hydrocarbonvapors having a substantially similar boiling point range. They may, onthe other hand, constitute selectively different groups of boiling pointranges. This may be governed by the use to be made of the overheadproducts from the expansion chambers or towers. For example, they may bemixed, blended, reprocessed, reformed, etc.

The higher boiling hydrocarbons which condense and collect in the towersare removed from the lower part of the respective towers through thedraw-ofi pipes 98, 9| and 92, all of which are shown as feeding intopipe 93 leading to sump 94. The mixed processed hydrocarbons whichcollect in this portion are pumped over through pipe I00, and aredelivered to the oil-vaporizing unit lili. From thence, the vapors aredelivered to the conversion zone |02 where, in contact with a suitablecatalytic agent and under predetermined thermal conditions, they aretreated and delivered to the dephlegmating or fractionating tower I03and permitted to expand. The lower boiling hydrocarbons areremoved fromtower I03 as vapor through pipe I34, and the heavier condensedhydrocarbons are removed from the tower through 1 pipe I and thencereturned to the sump from be :those of straight run gasoline, may bewithdrawn from the tower through the pipe I I5, and may then becondensed and collected or otherwise treated. The vapors of higherboiling hydrocarbons arepreferably fractionated within the tower andremoved selectively as side streams H3, Ill and H8, which, in turn, arevaporized in the oil heating furnaces II 9, I25 and I2I, respectively,at appropriate temperatures, the vapors being individually supplied tothe conversion zones I22, I23 and I24, respectively, for treatment underpredetermined thermal conditions. This treatment is preferably done inthe presence of a suitable catalyzer conditioned for each zone withparticular respect to the characteristics of the hydrocarbons passingtherethrough.

The heated vapors from zone I22 are delivered by pipe I25 to theexpansion chamber I3I, and the vapors from zone I23 are delivered bypipe 126 to the expansion chamber I32. Vapors from the zone I24, leavingby pipe I21, enter the heat exchanger :23 to supply heat to the coil I30therein. The vapors leaving heat exchanger I28 by pipe I29 enter theexpansion chamber I33. It will be noted that each expansion chamber isprovided with an overhead outlet pipe through which the vapors of lowboiling hydrocarbons may be removed. By means of pipe I35, higherboiling fractions condensing in chamber I3I are delivered into theexpansion chamber I32, preferably in direct and in indirect heatexchange relationship with the incoming heated conversion vapors fromzone l23. Likewise, the higher boiling fractions which condense in thechamber I32 are delivered into chamber I33 in heat-receivingrelationship to the heated vapors from I23.

The higher boiling hydrocarbons which condense and accumulate in I33 areremoved and delivered through pipe I43 to the oil heater I4I where theyare vaporized and delivered to the reaction zone I42 forthermo-catalytic conversion. The heated products from 142 then enter thetower I 43, from which tower lower boiling hydrocarbon vapors may beremoved through pipe I44. Higher boiling fractions which condense intower I43 are removed through pipe I45, delivered to the coil I30 forpreheating, and then through pipe I43 delivered into the tower I I4 inheat-exchange relationship with the incoming heated vapors from the coilIII. By means of the pipe I48, some of the condensed hydrocarbons fromI43 may be returned to the system for further treatment through thevalved inlet I49 leading to chamber I3I Also, by means of the valvedpipe branches I53 and I5I of the pipe I52, some or all of these heavierfractions may be delivered to one or both of the chambers I32 and I33,in a similar manner, for further'heating and treatment.

The several arrangements illustrated in the drawings for carrying outthe process of the present invention are illustrative only, and thesearrangements may obviously be modified without departing from the spiritor the invention, as defined by the appended claims; These severala-rran'gements have been selected to'show some of the methods ofcarrying the broad principle into practice.

An underlying principle has been clear- 'ly" set forth in theforegoingspecification; and it will beseen that the several variantseach accomplishes in its specific way the important feamm of theinventive idea. It will beu'nders't'ood, of course, that any suitablecatalyst may be selected for use in the several heat-treating zones,said catalysts or chemical promotors preferably being adaptedto theparticularoil being processed and/or to the particular fractionsselectively submitted to said thermal treatment. It will befurtherunderstood that in carrying out the process, suitable means will beemployed for temperature control, pressure control, or suchcombinations'of the .same as may be required.

1. In a process of selectively cracking a hydrocarbon oil stock, thesteps comprising preheating and vaporizing liquid hydrocarbon charge oilby indirect heat exchange with hot cracked vapors issuing from first andsecond thermal catalytic cracking zones, submitting the vapors of saidcharge oil to expansion and fractionation in a first fractionating zone,removing a lower boiling fraction as a vapor from said firstfractionating zone, condensing a higher boiling fraction in said firstfractionating zone, and subjecting the condensate, while still hot, tofurther treatment of direct evaporation and selective cracking in thefirst catalytic cracking zone for optimum conversion, expanding anddephlegmating the cracked Vapors in a second fractionating zone while inadmixture with the previously separated lower boiling point fractionremoved from the first fractionating zone, removing motor fuel vaporsfrom said second fractionating zone and condensing them, condensing aheavier fraction in said second fractionating zone, and vaporizing andsubjecting to further separate selective cracking said heavier fractionfrom the second fractionating zone in the second catalytic crackingzone, 00- mingling and expanding the cracked vapors therefrom with thoseobtained from the first cracking zone, fractionating the mixed crackedhydrocarbons in the second fractionating zone, and recycling the heaviercondensate therefrom through the second cracking zone.

2. In a system for selectively cracking a hydrocarbon oil stock, thecombination of a first catalytic cracking chamber and a second catalyticcracking chamber, a first heating element and a second heating elementassociated with said first and said second catalytic cracking chambers,respectively, for vaporizin oil stock prior to its entrance into eachchamber, a heat-exchange chamber receiving vapors from such crackingchambers, a heat-exchange element positioned within said chamber, pipemeans for delivering the charging stock to the heat-exchange elementwhereby the charging stock will be preheated and vaporized by indirectheat exchange from the cracked vapors from the catalytic chambers, afirst fractionating tower for receiving the preheated and vaporizedcharging stock, a second fractionating tower, pipe means connecting thefirst fractionating tower with the second fractionating tower fordelivery thereto of the lowboiling hydrocarbons from the first tower,means for withdrawing and condensing such lower-boiling hydrocarbonsfrom said second tower, means connecting the first tower with the firstof said 11 heating elements and the first cracking chamber, meansconnecting the second tower with the second heating element and thesecond cracking "chamber, and means connecting the heat-exchange chamberwith the second tower, whereby lower-boiling hydrocarbons from theheat-exchange chamber are removed and condensed, and higher-boilinghydrocarbons are recycled to the second heating element and secondcatalytic chamber.

EDWARD F. CHANDLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,972,149 Keith Sept. 4, 1934Houdry June 8, 1937 20 OTHER REFERENCES Eaton et a1., CriticalTemperatures Of Petroleum Oils; Ind. and Eng, Chem; vol. 24, No. 7; July1932, pp. 819-822.

Sachanen, Chemistry and Tech. 01 Cracking; 1932; pages 32 and 33. (Copy1n Div. 31, U. S. Patent Office.)

